For an optical disc, particularly a read-only ROM (Read Only Memory) disc, as an optical recording medium, a large amount of replica substrates can be produced at low costs in a short time by injection molding of plastic from a single stamper. Therefore, optical recording media are widely used as package media.
Incidentally, for preventing a copyright infringement, a method of adding identification information that differs for each disc is being considered. As the method of adding identification information, there is proposed a method of, for example, recording sub-data onto a metal reflective film (e.g., see Patent Document 1).
In an optical disc of Patent Document 1, an Ag (silver) alloy is used as the metal reflective film.
The metal reflective film formed of an Ag alloy can be used not only in a read-only optical recording medium of Patent Document 1 that records sub-data, but also in a general read-only optical recording medium.
However, since a silver alloy is somewhat expensive as a material of the read-only optical recording medium, a cost increases.
In this regard, using Al (aluminum) in place of Ag is being considered.
In actuality, Al or an Al alloy is used as a metal reflective film in an optical recording medium that records/reproduces information using light having wavelengths of 650 nm and 780 nm (see, for example, Patent Document 2).    Patent Document 1: Japanese Patent Application Laid-open No. 2007-335003    Patent Document 2: Japanese Patent Application Laid-open No. 2003-317318